The present invention relates to a system for monitoring the operation of rotary kilns and more particularly to a method and means for sensing temperatures within a rotary kiln, of the type such as used for the direct reduction of metal oxides using solid carbonaceous materials as both fuel and reductant, and displaying the sensed results at a remote location.
Temperature sensing and control is essential to the proper operation of a rotary kiln carrying out a direct reduction process such as the reduction of iron ores to directly reduced iron (DRI) or sponge iron using coal as the fuel and reductant. The temperatures within the kiln must be regularly sensed and accurately displayed to permit the kiln operators to supervise the process and initiate any necessary process changes or corrections promptly. The temperature sensors for this purpose have typically been thermocouples, disposed at spaced locations along the length of the kiln, which transmit low lever millivolt signal data from each location on the rotating kiln to a recorder in a central control room. The common technique has been to transmit the signals over fixed wires routed from each of the thermocouples along the kiln shell to a set of slip rings at a convenient central location on the kiln. The signals are then transferred from the slip rings through a set of sliding shoes at a stationary location adjacent the kiln and routed over further wires to the control room recorder. An example of a prior art system of this type is shown in U.S. Pat. No. 3,331,247 to Toepell wherein temperature indicative millivolt signals are generated from a number of thermocouples on a kiln, each of which thermocouples has one terminal connected to a common continuous slip ring mounted on the kiln, and another terminal connected to a separate segment of a segmented slip ring. A mechanically-actuated recorder switching system is also mounted on the kiln to permit transfer of the thermocouple signal voltages to an external receive in a sequential sampling sequence through brushes contacting the two slip rings.
These prior art systems have been plagued with a number of problems such as lack of temperature compensation of the signal at the slip rings junction, sliding contact noise and signal distortion, and long transmission line interference. Mechanical switching with physical contact has led to heavy maintenance requirements as the kiln expands and contracts and shifts longitudinally back and forth. Even with frequent calibrations and a thorough understanding of the system, accurate interpretation of data is difficult, and as calibration verification is normally accomplished with the kiln stopped, errors due to the sliding action between the rings in the stationary shoes during kiln rotation are not taken into consideration. There is, accordingly, a need in the art for an accurate, simple, easily-verified and calibrated and maintained kiln temperature measuring system.
The present invention is directed to providing such a temperature measuring system which eliminates many of the problems inherent in the prior art systems.